Internal combustion engines, and diesel engines in particular, are known to emit oxides of nitrogen under various operating conditions, which NOx may be hazardous to health. Emission limits promulgated by the US Federal government are stringent and are projected to become even more so. Accordingly, it is of great interest to engine and vehicle manufacturers to develop methods and apparatus for continuous removal of NOx from the exhaust streams of engines.
Efficient NOx traps are known in the prior art. Such traps are catalytic in nature and act to adsorb NOx from an exhaust stream passing over the catalytic surface. Periodically, however, the catalyst must be regenerated by exposure to a reductant, for example, diesel fuel or reformate. Thus, for continuous operation, it is known to provide first and second NOx traps arranged in parallel for alternative operation by a four-port diverter valve. While the first trap is being regenerated, the second trap is in service; when the second trap needs regeneration, the first trap is switched into service and the second into regeneration. Following regeneration, a typical prior art NOx trap has an effective adsorption lifetime of between about 25 seconds and 120 seconds. Regeneration requires typically about 5 seconds.
An expensive drawback of a prior art dual trap system is that each trap must be capable of treating the entire emission load by itself, and thus twice the required catalyst volume is needed to maintain targeted emission levels continuously. Further, each trap must occupy the minimum space required for treating the entire emission load.
A further disadvantage of a prior art dual trap system is that during regeneration a trap undergoes significant cooling and thus has lowered adsorption effectiveness for some period of time when coming up to operating temperature when placed back into service.
What is needed in the art is a means for reducing the overall size and cost of a continuously-operable NOx trap assembly.
What is further needed in the art is a means for minimizing the cooling which a trap undergoes while out of service for regeneration.
It is a principal object of the present invention to reduce the size and cost of a continuously-operable NOx trap assembly.
It is a further object of the present invention return a trap to service after minimal temperature loss.